Zeolite catalysts have become widely used in the processing of petroleum and in the production of various petrochemicals. Reactions such as cracking, hydrocracking, catalytic dewaxing, alkylation, dealkylation, transalkylation, isomerization, polymerization, addition, disproportionation and other acid catalyzed reactions may be performed with the aid of these catalysts. Both natural and synthetic zeolites are known to be active for reactions of these kinds.
The common crystalline zeolite catalysts are the alumino-silicates such as Zeolites A, X, Y and mordenite. Structurally, each such material can be described as a robust three dimensional framework of SiO.sub.4 and AlO.sub.4 tetrahedra that is crosslinked by the sharing of oxygen atoms whereby the ratio of total aluminum and silicon atoms to oxygen is 1:2. These structures (as well as other crystalline zeolites of catalytic usefulness) are porous, and permit access of reactant molecules to the interior of the crystal through windows formed of eight-membered rings (small pore) or of twelve-membered rings (large pore). The electrovalence of the aluminum that is tetrahedrally contained in the robust framework is balanced by the inclusion of cations in the channels (pores) of the crystal.
An "oxide" empirical formula that has been used to describe the above class of crystalline zeolites is EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
wherein M is a cation with valence n, x has a value of from 2 to 10, and y has a value which depends on the pore volume of the particular crystal structure under discussion. The empirical oxide formula may be rewritten as a general "structural" formula EQU M.sub.2/n [(AlO.sub.2).w(SiO.sub.2)]yH.sub.2 O
wherein M and y are defined as above, and wherein w has a value from 1 to 5. In this representation, the composition of the robust framework is contained within the square brackets, and the material (cations and water) contained in the channels is shown outside the square brackets. One skilled in the art will recognize that X in the empirical oxide formula represents the mole ratio of silica to alumina in the robust framework of a crystalline zeolite, and may be referred to herein simply by the expression in common usage, i.e. "the silica to alumina ratio". Further, the term framework, whenever used herein, is intended to refer to the robust framework described above. (See "Zeolite Molecular Sieves", Donald W. Breck, Chapter One, John Wiley and Sons, New York, N.Y. 1974, which is incorporated herein by reference as background material).
With few exceptions, (such as with Zeolite A wherein x=2) there are fewer alumina tetrahedra than silica tetrahedra in the robust frameworks of the crystalline zeolites. Thus, in general, aluminum represents the minor tetrahedrally coordinated constituent of the robust frameworks of the common zeolites found in nature or prepared by the usual synthetic methods.
For the above common zeolite compositions, wherein x has a value of 2 to 10, it is known that the ion exchange capacity measured in conventional fashion is directly proportional to the amount of the minor constituent in the robust framework, provided that the exchanging cations are not so large as to be excluded by the pores. If the zeolite is exchanged with ammonium ions and calcined to convert it to the hydrogen form, it acquires a large catalytic activity measured by the alpha activity test for cracking n-hexane, which test is more fully described below. And, the ammonium form of itself desorbs ammonia at high temperature in a characteristic fashion.
It is generally recognized that the composition of the robust framework of the synthetic common zeolites, wherein x=2 to 10, may be varied within relatively narrow limits by changing the proportion of reactants, e.g., increasing the concentration of the silica relative to the alumina in the zeolite forming mixture. However, definite limits in the maximum obtainable silica to alumina mole ratio are observed. For example, synthetic faujasites having a silica to alumina mole ratio of about 5.2 to 5.6 can be obtained by changing said relative proportions. However, if the silica proportion is increased above the level which produces the 5.6 ratio, no commensurate increase in the silica to alumina mole ratio of the crystallized synthetic faujasite is observed. Thus, the silica to alumina mole ratio of about 5.6 must be considered an upper limit for synthetic faujasite in a preparative process using conventional reagents. Corresponding upper limits in the silica to alumina mole ratio of mordenite and erionite via the synthetic pathway are also observed. It is sometimes desirable to obtain a particular zeolite, for any of several reasons, with a higher silica to alumina ratio than is available by direct synthesis. U.S. Pat. No. 4,273,753 to Chang and the references contained therein describe several methods for removing some of the aluminum from the framework by the use of aggressive treatments such as steaming, contact with chelating agents, etc., thereby increasing the silica to alumina ratio of a crystal. However, no generally useful method appears to have been described for increasing the alumina content of a zeolite crystal. Thus, although it is relatively easy to reversibly alter the composition of the materials (cations and water) contained within the channels of the crystalline zeolites, no generally useful method is known for reversibly altering the content of the minor tetrahedrally coordinated constituent in the structure of the robust framework.
Synthetic high silica content crystalline zeolites have been recently discovered wherein x is at least 12, some forms of these having little or even substantially no aluminum content. It is of interest that these zeolites appear to have no natural counterparts. These zeolites have many advantageous properties and characteristics such as a high degree of structural stability. They are used or have been proposed for use in various processes including catalytic processes. Materials of this type are known in the art and include high silica content aluminosilicates, such as ZSM-5 (U.S. Pat. No. 3,702,886), ZSM-11 (U.S. Pat. No. 3,709,979), and ZSM-12 (U.S. Pat. No. 3,832,449) to mention a few. Unlike the zeolites described above wherein x=2 to 5, the silica to alumina ratio for at least some of the high silica content zeolites is unbounded, i.e. the ratio may be infinitely large. ZSM-5 is one such example wherein the silica to alumina mole ratio is at least 12. U.S. Pat. No. Re. 29,948 to Dwyer et al. discloses a crystalline organosilicate essentially free of aluminum and exhibiting an X-ray diffraction pattern characteristic of ZSM-5 type aluminosilicates. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe microporous crystalline silicas or organosilicates with very low alumina contents. Some of the high silica content zeolites contain boron which is not reversibly removed by simple ion exchange or other non-aggressive means, i.e. the zeolites contain framework boron.
Because of the extremely low alumina content of certain high silica content zeolites, when such materials are converted to the ammonium form and calcined in the conventional manner to produce the hydrogen form, they are not as catalytically active as their higher alumina content counterparts. In copending U.S. patent application Ser. No. 391,212 filed June 23, 1982 (now abandoned), a method is described for enhancing the acid activity of a high silica content crystalline zeolite having substantially no acid activity, by compositing said zeolite with an acidic inorganic oxide under prescribed conditions.
It is an object of the present invention to provide an improved method for increasing the acidic catalytic activity of a high silica content zeolite that contains framework boron. It is a further object of this invention to provide a method for substituting aluminum for boron contained in the robust framework of a high silica content zeolite. It is a further object of this invention to provide novel catalytic compositions prepared by the method of this invention.